1. Field of the Invention
The present invention relates generally to high temperature Fischer-Tropsch technology and, more particularly, to a preparation of iron/carbon (Fe/C) nanocomposite catalysts for Fischer-Tropsch synthesis reaction and a related production of liquid hydrocarbons.
2. Description of the Related Art
Fischer-Tropsch (hereinafter abbreviated to FT) synthesis technology was first developed by two German chemists Franz Fischer and Hans Tropsch in the 1920s. The FT process produces synthetic fuels (i.e., hydrocarbon) from synthetic gas (i.e., hydrogen and carbon monoxide) through the following reaction: (2n+1)H2+nCO→CnH(2n+2)+nH2O. In the FT synthesis reaction that usually uses cobalt- or iron-based catalysts, reaction conditions such as a reaction temperature, a reaction pressure and gas composition depend on the type of catalyst used. Such FT synthesis reactions are often divided, according to their operation temperature, into low temperature FT (LTFT) operating at 200˜250° C. and high temperature FT (HTFT) at 300˜350° C. (Andrei Y. Khodakov et al, Chem. Rev., 2007, 107, 1672). Typically, in case of LTFT, cobalt-based catalysts with a relatively longer lifespan are used.
Cobalt-based catalysts have disadvantages such as the risk of poisoning caused by sulfur compounds and relatively high cost, while having advantages such as high activity and long lifespan. Further, cobalt-based catalysts have little activity to the water-gas shift (WGS) reaction, so that the ingredient ratio of synthetic gas (hydrogen and carbon monoxide in the proportion 2:1) exerts a strong influence on the FT reactions. On the contrary, iron-based catalysts having activity to the water-gas shift reaction can be used as various compositions in which the ingredient ratio of hydrogen and carbon monoxide varies between 1 and 2, and further used even under the existence of carbon dioxide. Therefore, in case of the high temperature FT reaction which is commercially applied on a large scale, iron-based catalysts have been usually used due to their advantages such as low cost and less weak sulfur tolerance.
Iron-based catalysts used for the low temperature FT reaction have been often prepared using a co-precipitation method (Korean Patent No. 10-1087165 titled “The method for preparing of Fe based catalyst used in Fischer-Tropsch synthesis reaction and that of liquid hydrocarbon using Fe based catalyst”). This catalyst has merits such as high iron content by weight with regard to total catalyst, but has several demerits such as complicated preparation process, low reliability, catalyst coking due to carbon monoxide, and poor stability at a high temperature. On the contrary, the high temperature FT reaction applied to produce light naphtha or gasoline has usually used fused Fe particles in a commercial process, and also has often used supported catalysts using supports in a laboratory-scale research stage (Qinghong Zhang et. al., ChemCatChem 2010, 2, 1030). Fused Fe applied to a commercial process has high mechanical strength because it is prepared through a melting process at a very high temperature more than 1000° C. However, it has been known as having some demerits such as a small crystal size and low activity (Hiroshige Matsujioto et. al., J. Catal. 1978, 53, 331).
Supported catalysts have been initially prepared through a wetness-impregnation method as disclosed in Korean Patent Publication No. 10-2011-0109625 titled “A noble catalyst of aqueous phase reforming reaction using mesoporous alumina carrier and platinum and manufacturing method thereof” and Korean Patent Publication No. 10-2011-0109624 titled “A noble catalyst of aqueous phase reforming reaction using mesoporous carbon carrier and multi-component metal and manufacturing method thereof”. However, such wet type methods not only should select a suitable solvent for each metal salt to obtain catalysts with uniformly dispersed active particles, but also repeatedly require absorption and dry steps. Unfortunately, this process using a solvent may often invite the danger of work, environmental pollution, and a burden of solvent treatment at the mass production of catalysts.
Additionally, in case of conventional catalysts, it is required to activate catalyst particles of inactive iron oxide through a reduction process in a reactor. Therefore, a longer activation time of about 24 hours is unfavorably needed, and also such reduction or activation conditions negatively affect the activity of catalyst (Dragomir B. Bukur, J. Catal. 1995, 155, 353).